1. Field of the Invention
The present invention relates to a timing recovery apparatus using an equalizer of an E2PR4ML method and a method therefor and an apparatus for judging last data and, more particularly, to a timing recovery apparatus for providing effective and stable timing to a sampling unit using an E2PR4ML signal which is a target signal of a magnetic recording equalizer and a method therefor and an apparatus for judging last data using a new target signal generated by the timing recovery apparatus.
2. Description of Related Art
A PR4ML method is an equalizer method currently used in a magnetic recording channel method to deal with high density recording. During a super-high density recording, an E2PR4ML equalizing method is used since there are limitations in the performance of a system that uses the PR4ML method. Since the E2PR4ML equalizing method has a frequency characteristic that is more similar to the channel characteristic during super-high recording than the conventional PR4ML equalizing method, noise amplification in an equalizer output decreases. Accordingly, it is possible to obtain a system having better performance. The operation of a timing recovery apparatus for a recovering sampling clock, however, is much more unstable in the E2PR4ML method than in the PR4ML method. This is because a method of using the output value of the equalizer and a threshold detection value with respect to the output value is employed in a PRML affiliated timing recovery apparatus. Specifically, the number of data levels in the output of the equalizer is 3 in the case of the PR4ML method, and the number of data levels in the output of the equalizer is 7 in the case of the E2PR4ML method. Therefore, the probability of error of the threshold detection value is much higher in the case of the E2PR4ML method than in the case of the PR4ML method. Accordingly, the operation of the timing recovery apparatus using the threshold detection value of the E2PR4ML method becomes unstable.
FIG. 1 shows a conventional timing recovery apparatus using the E2PR4ML method and an apparatus for judging last data using a target signal generated by the timing recovery apparatus. Reference numeral 10 denotes an apparatus for judging last data. Reference numeral 12 denotes a timing recovery apparatus. Reference numeral 100 denotes a sampling unit for sampling a reproduced signal including a low-pass filter 1002 and an analog/digital (A/D) converter 1004. Reference numeral 102 denotes an equalizer for receiving a signal output from the sampling unit 100 and filtering it so as to output a target signal H(D) E2PR4ML. Reference numeral 104 denotes a first Viterbi decoder for judging data using the target signal H(D) of the equalizer 102. Reference numeral 106 is a timing recovery unit for detecting the timing error of the target signal using judged data output from the first Viterbi decoder 104, synchronizing a phase as much as the detected error signal, and providing the synchronized phase as a clock signal of the sampling unit 100. Reference numeral 110 denotes a second Viterbi decoder for judging last data using the target signal H(D) of the equalizer 102.
Continuously, the operations of the timing recovery apparatus 12 and the last data judging apparatus 10 shown in FIG. 1 will now be described.
The band of an input reproduced signal is restricted through a low-pass filter 1002. The input reproduced signal is converted into a digital signal through an analog/digital converter 1004 and is input to the equalizer 102. The E2PR4ML signal which is the target signal H(D) of the equalizer 102 is generated by filtering the signal input through the equalizer 102.
The operation of the timing recovery apparatus 12 using the E2PR4ML which is the target signal H(D) of the equalizer 102 generated as mentioned above follows. The timing recovery unit 106 detects the timing error of the sampling clock by receiving the output signal of the equalizer 102 and the output signal of the first Viterbi decoder 104, synchronizes a phase as much as the detected timing error, and provides the synchronized phase as a clock signal of the sampling unit 100.
The operation of the last data judging apparatus 10 for judging the last data using the E2PR4ML signal output from the equalizer 102 follows. The second Viterbi decoder 110 which receives the E2PR4ML which is the target signal H(D) of the equalizer 102 judges and outputs the last data.
However, since the timing recovery apparatus in the conventional E2PR4ML method stably operates in only a very narrow range, the timing recovery apparatus operates unstably when the timing error is greater than a predetermined degree. Accordingly, the timing error cannot be completely removed. Thus, the performance of the timing recovery apparatus in the E2PR4ML method is lower than in the PR4ML method.
It is difficult to safely employ the E2PR4ML method in a system since the performance of the timing recovery apparatus using the E2PR4ML method is much lower than in the PR4ML method. The E2PR4ML method, however, has much less noise amplification than the conventional PR4ML method and has been used as an equalizer in a high density magnetic recording channel or a communication channel in which inter-symbol interference is severe.